1. Field of the Invention
The present invention relates generally to an improvement in a hydraulic control system for a transmission which is employed in industrial vehicles such as, for example, fork lifts and shovel loaders and which is equipped with a hydraulically operated clutch of a wet type and particularly to an improvement in an inching valve forming a part of the hydraulic control system.
2. Description of the Prior Art
As is well known in the art, in a hydraulic control system of this type a selector valve is fed with a hydraulic fluid pressure which is produced by a pump and is regulated by a pressure regulator valve. The regulated hydraulic fluid pressure is fed from the selector valve operated by the driver through a selector lever to a hydraulic fluid chamber of a selected section of the clutch connected to a torque converter to perform forward or reverse travel or a halt of the vehicle.
Furthermore, an inching valve interlocked with the brake pedal of the vehicle is arranged between the pressure regulator valve and the selector valve. The inching valve serves, when the brake pedal is depressed, to reduce the hydraulic fluid pressure fed to the clutch to automatically disengage it before the brakes are acting even if the selector valve is in a position for the forward or reverse travel of the vehicle. The inching valve serves, when the brake pedal is released, to engage the clutch.
An inching valve of a conventional hydraulic control system has had a structure which causes the hydraulic fluid pressure fed to the clutch to be abruptly varied at a stage of and with respect to the displacement of the inching valve. As a result, since range of the displacement of the inching valve in which the clutch is incompletely engaged is very narrow, it has been difficult or impossible to gently or slowly engage the clutch to effect an inching operation of the vehicle.